1. Field of the Invention
This invention relates to field emission display (FED) devices. More particularly, this invention relates to methods and apparatuses for improving beamlet uniformity in FED devices.
2. Description of the Related Art
Field emission display (FED) devices are an alternative to cathode ray tube (CRT) and liquid crystal display (LCD) devices for computer displays. CRT devices tend to be bulky with high power consumption. While LCD devices may be lighter in weight with lower power consumption relative to CRT devices, they tend to provide poor contrast with a limited angular display range. FED devices provide good contrast and wide angular display range and are lightweight with low power consumption. An FED device typically includes an array of pixels, wherein each pixel includes one or more cathode/anode pairs. Thus, it is convenient to use the terms “column” and “row” when referring to individual pixels or columns or rows within the array.
FIG. 1 illustrates a portion of an FED device 10 produced in accordance with conventional micro-tipped cathode structure. The FED device 10 includes a faceplate 12 and a baseplate 20, separated by spacers 32. The spacers 32 support the FED device 10 structurally when the region 34 in between the faceplate 12 and the baseplate 20 is evacuated. The faceplate 12 includes a glass substrate 14, a transparent conductive anode layer 16 and a cathodoluminescent layer or phosphor layer 18. The phosphor layer 18 may include any known phosphor material capable of emitting photons in response to bombardment by electrons.
The baseplate 20 includes a substrate 22 with a row electrode 24, a plurality of micro-tipped cathodes 26, a dielectric layer 28 and a column-gate electrode 30. The baseplate 20 is formed by depositing the row electrode 24 on the substrate 22. The row electrode 24 is electrically connected to a row of micro-tipped cathodes 26. The dielectric layer 28 is deposited upon the row electrode 24. A column-gate electrode 30 is deposited upon the dielectric layer 28 and acts as a gate electrode for the operation of the FED device 10.
The substrate 22 may be comprised of glass. The micro-tipped cathodes 26 may be formed of a metal such as molybdenum, or a semiconductor material such as silicon, or a combination of molybdenum and silicon. Micro-tipped cathodes 26 may also be formed with a conductive metal layer (not shown) formed thereon. The conductive metal layer may be comprised of any well-known low work function material.
The FED device 10 operates by the application of an electrical potential between the column electrode 30 or gate electrode 30 and the row electrode 24 causing field emission of electrons 36 from the micro-tipped cathode 26 to the phosphor layer 18. The electrical potential is typically a DC voltage of between about 30 and 110 volts. The transparent conductive anode layer 16 may also be biased (1-2 kV) to strengthen the electron field emission and to gather the emitted electrons toward the phosphor layer 18. The electrons 36 bombarding the phosphor layer 18 excite individual phosphors 38, resulting in visible light seen through the glass substrate 14.
The micro-tipped cathodes 26 of FED device 10 are three-dimensional structures which may be formed as evaporated metal cones or etched silicon tips. Micro-tipped cathodes 26 provide enhanced electric field strength by about a factor of four or five over the two-dimensional structure of the two-dimensional alternative FED device 40 (see FIG. 2). However, the two-dimensional structure of the alternative FED device 40 can be formed with planar films and photolithography.
Referring to FIG. 2, a portion of an alternative FED device 40 is shown in accordance with conventional flat cathode structure. FED device 40 includes a faceplate 42 and a baseplate 50 separated by spacers (not shown for clarity). The faceplate 42 may include a glass substrate 44, a transparent conductive anode layer 46 disposed over the glass substrate 44, and a phosphor layer 48 disposed over transparent conductive anode layer 46. An electrical potential of between about one kilovolt to about two kilovolts may be applied to the transparent conductive anode layer 46 to enhance field emission of electrons and to gather emitted electrons at the phosphor layer 48.
The baseplate 50 may include a substrate 52, a conductive layer 54, a flat cathode emitter 56, a dielectric layer 58 and a grid electrode 60. The conductive layer 54 may be a row electrode 54 and is deposited on the substrate 52. The flat cathode emitter 56 and dielectric layer 58 are deposited on the conductive layer 54. The grid electrode 60 may also be referred to as the column electrode 60. The grid electrode 60 is deposited over, and supported by, the dielectric layer 58. The flat cathode emitter 56 may comprise a low effective work function material such as amorphic diamond.
Several techniques have been proposed to control the brightness and gray scale range of FED devices. For example, U.S. Pat. No. 5,103,144 to Dunham, U.S. Pat. No. 5,656,892 to Zimlich et al. and U.S. Pat. No. 5,856,812 to Hush et al., incorporated herein by reference, teach methods for controlling the brightness and luminance of flat panel displays. However, even using these brightness control techniques, it is still very difficult to obtain a uniform electron beam from an FED emitter. Thus, there remains a need for methods and apparatuses for controlling FED beam uniformity.